git-clean: honor core.excludesfile

git-clean did not honor core.excludesfile configuration
variable, although some other commands such as git-add and
git-status did.  Fix this inconsistency.

Original report and patch from Shun'ichi Fuji.  Rewritten by me
and bugs and tests are mine.

Signed-off-by: Junio C Hamano <gitster@pobox.com>